Digital image storage is a process whereby an image is stored in a storage medium as a plurality of bits representing pixels (picture elements) of a given intensity. Digital storage offers a number of distinct advantages to image storage and transmission, image processing, image retrieval etc. over the more traditional hard copy. From the point of ready access, a computerized database, or even bulk storage on disk is easier to manipulate than paper folders containing film sheets. Image transmission over telephone lines to remote locations becomes possible. It is not therefore surprising that digital image capture, conversion and storage, finds great utility in applications where there is need for such advantages.
Applications are found in fields otherwise as diverse as medical diagnostic radiology and the publishing industry, where electronic page composition is becoming increasingly common. Often in such applications there is need to alter the original size of a given image data to fit within prescribed space limitations. These limitations are imposed by a number of extraneous factors which are usually the result of physical constraints in the output product, e.g., a sheet of film or paper on which one or more images are displayed. For instance, in a medical diagnostic application, it may be advantageous to display a number of different radiograms, side by side on a single radiological film for a physician to examine. Similarly, a number of distinct images and corresponding text may have to be fitted on a sheet of paper forming a magazine page. In both instances, it is rare that the original images all happened to be the exact size that will fit in the available space. In most cases the stored image size will have to be altered to a new size commensurate with the space it is to occupy in the display medium.
One possible solution, albeit cumbersome and slow, is to generate an image that is an exact duplicate of the original using the digital data, and then project this image using variable magnification lens systems onto a portion of a photosensitive film sheet. By varying the magnification the projected image can be made to fill the allocated space. The process is then repeated for each of the images and sizes used until the film sheet is filled.
One would rather, however, employ electronic means which operate directly and rapidly on the digital data to provide the displayed image data in proper format so that the displayed image fits exactly in the allocated space. That is one would rather have direct, electronic image magnification for image fitting and the ability to do so with or without altering the original image aspect ratio as desired. The image data could then be used to drive an output device which may be a laser printer scanner such as described in U.S. Pat. Nos. 4,080,634 or 4,067,021 or a CRT exposure tube. Similarly other printers employing a thermal head of the type disclosed in U.S. Pat. No. 3,951,247 may be used and the like.
A typical image produced by such device, is generated by writing with a moving spot of controlled (modulated) energy on a sensitive film sheet, point by point, along sequential raster lines, spots of variable density which correspond to the original pixel information that was extracted from an original image and stored in a storage medium.
This type of output display device is well known in the art, and need not be elaborated upon here, but will be referred to hereinafter as a recorder, (regardless of whether the raster is generated using electrical, mechanical or combination methods, and without regard to the nature of the writing source, provided such source is amenable to some form of output modulation control which alters the resulting density of the writing on an output film sheet).
To reproduce an original picture in exactly the same size, one simply selects a recorder having a spot size the same size as that of the scanner first used to acquire the stored image pixels. Thus a 3 inch by 3 inch rectangular image acquired with a scanner having a resolution of 300 pixels per inch in two orthogonal directions extending along X (horizontal) and Y (vertical) directions co-extensive with the image borders, can be reproduced exactly in a 3 inch by 3 inch space using a recorder capable of writing 300 spots and 300 lines per inch. The fitting of the original picture in the allotted space will be straightforward, one pixel to one spot and no data will be lost or artifacts created. However, if, for example, the original image is composed of 900 pixels.times.900 raster lines, which must be fitted in a 2.times.2 inch space using a recorder having a resolution of 1000 spots/inch and 1000 raster lines/inch, the resulting output image size in terms of spots and lines equals 2000.times.2000 spots for which we to provide intensity values. Therefore there is insufficient data from the original to fill the excess spots.
One solution used in the prior art is to generate fictitious pixel values from those on hand. The simplest way in this instance is to double the original pixels by replicating each at least once so that we now have an original image composed of 1800.times.1800 pixels and lines. This, however, only works for magnifications that are exact multiples of the original. Another approach is to try to predict values to generate imaginary pixels to fill the voids as shown in U.S. Pat. No. 4,595,958. However, there is a problem in the creation and allocation of pixels for the partial magnification, i.e., the need to magnify the image 2000/900=2.222 times. The generation of data for, and the distribution of, the extra 200.times.200 spots in the output image end up creating artifacts and/or image fuzziness in the final product.
U.S. Pat. No. 4,080,634 briefly discusses a method to scale original analog images for digital recording. According to its teachings, the scanning (reading) rate of an original image may be changed so that by the time the spot reaches the end of each line, a preselected number of samples have been taken for that line. At the same time, the rate of advancement of the image under the scanning line is modified so that the vertical scan distance compensates for the horizontal reading rate to maintain the original image aspect ratio.
The problem with such a process as applied to recording, is that the residence time of the spot per unit area changes, which in turn changes the exposure of the writing or recording medium. For small changes in magnification, this is not a problem, but for changes such as 2, 4, 8, or 10 times magnification, one starts encountering exposure problems, due to reciprocity failure and incorrect exposure in photosensitive systems, or thermal delay in thermally activated systems. The lines in the vertical direction become separated and the spaces between lines start becoming visible. There is still need for an effective image altering system capable of fitting an arbitrary digitally stored image retrieved from a storage medium into an arbitrary fixed allocated area in a display medium, which will not result in loss of information or introduction of artifacts or distortion.